dt-bindings: input: atmel,maxtouch: add wakeup-source

The touchscreen can be a wake up source and it's being used in DTS:

  arch/arm/boot/dts/exynos5250-spring.dt.yaml:
    trackpad@4b: 'wakeup-source' does not match any of the regexes: 'pinctrl-[0-9]+'

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210212163806.69996-1-krzk@kernel.org
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>